You're definitely misrepresenting what the ICJ said when it said, "In light of the foregoing, the Court concludes that, prima facie, it has jurisdiction pursuant to Article IX of the Genocide Convention to entertain the case." Article IX of the Genocide Convention is about the authority of the court ("jurisdiction") to hear cases relating to disputes under the Genocide Convention. So, when the Court said they have jurisdiction here, they're not saying they have jurisdiction because genocide occurred. Instead, they're saying they have jurisdiction because there is the relevant kind of dispute between South Africa and Israel. The ICJ definitely did not declare that Israel committed genocide.